[FEATURE] Improved title rendering

The title is now rendered using the lib.stdheader configuration. The
header_layout field of the plugin content element can now be used to
change the header layout of the poll.
[TASK] Simplified config handling

A new method getConfigValue() was added to the tx_jkpoll_pi1 class that
merges the TypoScript and the FlexForm configuration.

This method is now used for reading configuration and some parts
additionally were simplified and duplicate code was removed.
[TASK] More whitespace fixes
[TASK] Added / improved some comments
[FEATURE] Poll diplay mode also displays result

To simplify the usage and configuration of the extension the results
can also be displayed if what_to_display is set to POLL or not set
at all.
[TASK] Removed another clearing div
[FEATURE] Advanced error handling

If a user is not able to vote he will get an error message containing
information why he can not vote.